Operating System Model

In most multiuser operating systems, applications are separated from the operating system itself the operating system kernel code runs in a privileged processor mode (referred to as kernel mode in this book), with access to system data and to the hardware; application code runs in a nonprivileged processor mode (called user mode), with a limited set of interfaces available, limited access to system data, and no direct access to hardware. When a user-mode program calls a system service, the processor traps the call and then switches the calling thread to kernel mode. When the system service completes, the operating system switches the thread context back to user mode and allows the caller to continue.

Windows is similar to most UNIX systems in that it's a monolithic operating system in the sense that the bulk of the operating system and device driver code shares the same kernelmode protected memory space. This means that any operating system component or device driver can potentially corrupt data being used by other operating system components.

Is Windows a Microkernel-Based System?

Although some claim it as such, Windows isn't a microkernel-based operating system in the classic definition of microkernels, where the principal operating system components (such as the memory manager, process manager, and I/O manager) run as separate processes in their own private address spaces, layered on a primitive set of services the microkernel provides. For example, the Carnegie Mellon University Mach operating system, a contemporary example of a microkernel architecture, implements a minimal kernel that comprises thread scheduling, message passing, virtual memory, and device drivers. Everything else, including various APIs, file systems, and networking, runs in user mode. However, commercial implementations of the Mach microkernel operating system typically run at least all file system, networking, and memory management code in kernel mode. The reason is simple: the pure microkernel design is commercially impractical because it's too inefficient.

Does the fact that so much of Windows runs in kernel mode mean that it's more susceptible to crashes than a true microkernel operating system? Not at all. Consider the following scenario. Suppose the file system code of an operating system has a bug that causes it to crash from time to time. In a traditional operating system, a bug in kernelmode code such as the memory manager or the file system would likely crash the entire operating system. In a pure microkernel operating system, such components run in user mode, so theoretically a bug would simply mean that the component's process exits. But in practical terms, the system would crash because recovering from the failure of such a critical process would likely be impossible.

All these operating system components are, of course, fully protected from errant applications because applications don't have direct access to the code and data of the privileged part of the operating system (although they can quickly call other kernel services). This protection is one of the reasons that Windows has the reputation for being both robust and stable as an application server and as a workstation platform yet fast and nimble from the perspective of core operating system services, such as virtual memory management, file I/O, networking, and file and print sharing.

The kernel-mode components of Windows also embody basic object-oriented design principles. For example, they don't in general reach into one another's data structures to access information maintained by individual components. Instead, they use formal interfaces to pass parameters and access and/or modify data structures.

Despite its pervasive use of objects to represent shared system resources, Windows is not an object-oriented system in the strict sense. Most of the operating system code is written in C for portability and because C development tools are widely available. C doesn't directly support object-oriented constructs, such as dynamic binding of data types, polymorphic functions, or class inheritance. Therefore, the C-based implementation of objects in Windows borrows from, but doesn't depend on, features of particular object-oriented languages.